Magnetic metal powder suitable for use in magnetic recording media and method of manufacturing the powder

ABSTRACT

A metal magnetic powder for a magnetic recording medium is provided whose particles have a metal magnetic phase, composed mainly of Fe or Fe plus Co, and an oxide layer, wherein the average major axis length of the powder particles is 10-50 nm, the average particle volume including the oxide layer is 5,000 nm 3  or less, the atomic ratio (R+Al+Si)/(Fe+Co) calculated using the content values (at. %) of the elements contained in the powder particles is 20% or less, where R is rare earth element (Y being treated as a rare earth element). The metal magnetic powder is obtained by using a complexing agent and a reducing agent to elute nonmagnetic constituents after firing. The metal magnetic powder exhibits a large saturation magnetization σs for its particle volume while maintaining weatherability comparable to the conventional level and is suitable for a coated-type magnetic recording medium.

FIELD OF THE INVENTION

This invention relates to a ferromagnetic metal powder for use incoated-type magnetic recording media and a method of manufacturing thepowder.

BACKGROUND ART

Magnetic recording media, typically those used for backing up computerdata, require enhanced recording density to meet increasing storagecapacity requirements. A magnetic powder of small particle volume isconsidered necessary for achieving high recording density. Assignee hasresponded to these circumstances by establishing the manufacturingmethod set out in Patent Document No. 1 shown below and developingvarious other improvements in metal magnetic powder productiontechnology.

As can also be seen in the literature published heretofore, the mainconstituent of metal magnetic powders is typically iron. Industrialmanufacture of iron-system metal magnetic powder is generally carriedout by the method of incorporating a sintering inhibitor such as Si, Al,rare earth element or an alkaline earth metal element into an acicularpowder comprised mainly of iron oxy-hydroxide or iron oxide and thenreducing the powder.

Earlier attempts to enhance magnetic powder properties have focusedmainly on how to improve the properties of the magnetic powder itselfand on how to prevent sintering and thus improve dispersibility. PatentDocuments Nos. 2 to 5 define the atomic ratio of rare earth element andthe like on the particle surface and teach that magnetic recording mediaexcellent in electromagnetic conversion characteristics can be obtainedby using magnetic particles falling with the defined ranges. Ofparticular interest is Patent Document No. 5, which defines the amountof sintering inhibitor per unit surface area and teaches that inclusionof sintering inhibitor at or greater than the prescribed value isnecessary to prevent particle adhesion and enhance dispersibility, andthereby improve the magnetic properties and surface properties of themagnetic recording medium.

In order to boost the recording density of a magnetic recording medium,it is necessary to increase the number of magnetic particles includedper unit volume and therefore necessary to reduce particle size. PatentDocument Nos. 7 and 8, for example, teach use of a magnetic powder whoseparticles are reduced to the finest possible. The particle-size refiningmethod used is generally to carry out particle-size refinement at thestage of the starting material (precursor) for synthesizing magneticparticles by firing and reduction. However, when the refined precursoris subjected to firing and reduction, the likelihood of inter-particlesintering, axial ratio degradation owing to particle shapedeterioration, and other such problems tends to increase. This has madeit necessary to include a large amount of sintering inhibitor in theprecursor.

However, the rare earth element, Al and Si used as sintering inhibitorsare nonmagnetic, so that increasing the relative content of these“nonmagnetic constituents” per unit volume of the magnetic powder lowersthe saturation magnetization. As explained above, the need to increasethe amount of sintering inhibitor arises particularly when the particlesize of the magnetic powder is refined, so that there has been a problemof the saturation magnetization being markedly degraded owing toincrease in the amount of sintering inhibitor per unit volume. Further,Patent Document No. 9 teaches a method of increasing dispersibility bysubjecting the magnetic particles to compression deaeration fordecoupling the bonds caused by sintering and inter-particle action.

On the other hand, reduction of particle volume (size) by refinementgenerally lowers saturation magnetization. One reason that can be givenfor this is that the particle surface needs to be formed with an oxidelayer of a certain thickness for maintaining the weatherability of themagnetic powder, so that the percentage of particle volume accounted forby the metal component decreases with higher particle-size refinement.An effective way to improve the magnetic properties of a metal magneticpowder is to increase the percentage of the volume accounted for by themetal portion, i.e., the magnetized portion, and this has been a commonavenue of approach in the past. However, the method employed focuses onregulating the thickness of the oxide layer, and in this case a problemremains in that weatherability is degraded owing to the fact that therelative thickness of the oxide layer diminishes.

Patent Document No. 1: JPA-07-022224 Patent Document No. 2:JPA-06-215360 Patent Document No. 3: JPA-07-078331 Patent Document No.4: JPA-07-184629 Patent Document No. 5: JPA-2003-296915 Patent DocumentNo. 6: JPA-2005-101582 Patent Document No. 7: JPA-2003-242624 PatentDocument No. 8: JPA-2005-259929 Patent Document No. 9: Japanese PatentNo. 3043785 OBJECT OF THE INVENTION

As pointed out in the foregoing, when magnetic powder particle-sizerefinement is attempted, a problem of adverse effect on magneticproperties tends to arise owing to a relative increase in the amount ofadded sintering inhibitor and a relative increase in the amount of oxidelayer required for maintaining weatherability. This problem is verydifficult to overcome by a direct method of reducing the amount of addedsintering inhibitor and/or reducing the thickness of the oxide layer.

Moreover, realization of better tape orientation requires dispersibilityenhancement through sinter prevention. However, pursuit of a higherlevel of particle-size refinement makes sintering more likely to occurowing to the resulting increase in the surface area of the magneticpowder. A particularly notable finding that emerged from recent studiesby the inventors is that while a certain degree of sinter preventioneffect can be realized by inhibiting inter-particle sintering by use ofa method like that set out in Patent Document No. 5, it is difficult toachieve total prevention of inter-particle sintering simply byincreasing the amount of sintering inhibitor, especially in the case ofa magnetic powder comprising fine particles of a particle diameter of100 nm or less. In addition, dispersibility is degraded by the presenceof large particle clusters formed by sintering, giving rise to a problemof tape orientation property degradation, while the increase in theamount of sintering inhibitor raises the nonmagnetic constituentcontent, thus reducing the magnetic constituent content per unit volumeof the magnetic powder and causing a problem of Br (residual magneticflux density) decrease at the time of tape-making.

A method such as that set out in Patent Document No. 9 of breaking upclusters by compression has a problem in that it degrades magneticproperties by applying damage-causing mechanical pressure to themagnetic particles. This method was determined to be disadvantageousbecause the tendency to cause damage is particularly pronounced in thecase of a magnetic powder comprising fine particles of a particlediameter of 100 nm or less. To be more specific, although pastexperience demonstrates that a magnetic powder comprising fine particlesof a particle diameter of 100 nm or less requires reduction ofnonmagnetic sintering inhibitor content and improvement ofdispersibility, it is further true that reducing sintering inhibitorcontent promotes sintering between the particles and that this tendencyis more pronounced for fine particles. Therefore, the conventionalpractice has if anything been to increase the amount of added sinteringinhibitor with increasing particle-size refinement and it has not beenreadily possible to reduce the amount of added sintering inhibitor,i.e., to adopt what would be an ideal method.

It is worth noting that the sintering inhibitor is added to the startingmaterial before firing for the purpose of sinter prevention duringfiring and high-temperature reduction and has no further role to play atthe stage where it is present in the metal magnetic powder synthesizedby the firing and high-temperature reduction. Therefore, if instead ofreducing the amount of added sintering inhibitor it should be possibleto remove the “nonmagnetic constituents” originating from the sinteringinhibitor and present in the metal magnetic powder, it would be possibleto increase the relative content of the metal portion, i.e., themagnetized portion, while also achieving sinter prevention, and therebyinhibit decease of saturation magnetization with particle-sizerefinement. Further, one cause of adhesion of dirt for magnetic headwhen using a coated-type magnetic recording media is thought to besintering inhibitor-originating components present on the particlesurfaces, so removal of the nonmagnetic constituents would also be aneffective measure from this viewpoint.

In addition, the sintering inhibitor segregates near the surface of themagnetic powder. The heat during firing and reduction fuses thesintering inhibitor to cause adhesion (necking) of the sinteringinhibitor components at the individual particle surfaces and themagnetic particles form clusters as a result. Since removal of thesintering inhibitor near the surface would therefore minimize neckingand decrease particle cluster formation, the magnetic powder could beexpected to exhibit improved dispersibility in a binder.

However, effective removal of only sintering inhibitor-originatingnonmagnetic constituents from the particles of the metal magnetic powderis not necessarily easy and no method for this purpose has beenestablished heretofore.

In the light of this situation, the object of the present invention isto provide a technology for removing from the particles of a metalmagnetic powder nonmagnetic constituents originating from, sinteringinhibitor that has fulfilled its purpose, by this provide a metalmagnetic powder that exhibits a large saturation magnetization σs(Am²/kg) for its particle volume while maintaining weatherabilitycomparable to the conventional level and is suitable for a coated-typemagnetic recording medium, and provide a coated-type magnetic recordingmedium using the metal magnetic powder.

SUMMARY OF THE INVENTION

The foregoing object is achieved by a metal magnetic powder for amagnetic recording medium whose particles have a metal magnetic phase,composed mainly of Fe or Fe plus Co, and an oxide layer, wherein theaverage major axis length of the powder particles is 10-50 μm, theaverage particle volume including the oxide layer is 5,000 nm³ or less,the atomic ratio (R+Al+Si)/(Fe+Co) calculated using the percentage ofcontent (at. %) of the elements contained in the powder particles is 20%or less, and the content of the nonmagnetic constituents is preferablynot greater than 40 μmol/m² per unit surface area of the powder.

“Atomic ratio (R+Al+Si)/(Fe+Co) is 20% or less” means that the followingFormula (2):

(R+Al+Si)/(Fe+Co)×100≦20  (2)

is satisfied when each element symbol is replaced with the content ofthe corresponding element expressed in at. %. R represents rare earthelement (Y being treated as a rare earth element). Rare earth element,Al and Si need not all be contained. If an element is not contained, thecorresponding element symbol in Formula (2) is replaced with zero.

Rare earth element (Y being treated as a rare earth element), Al and Siare called nonmagnetic constituents and “the content of the nonmagneticconstituents is not greater than 40 μmol/m² per unit surface area of thepowder” means that the following Formula (3) is satisfied.

[Total number of mols of R+Al+Si per gram of powder] (μmol/g)/[PowderBET specific surface area] (m²/g)≦40  (3)

The metal magnetic powder according to the present invention meets thefollowing relationship when its pore size distribution is measured bythe mercury penetration method. The relationship is such that thecumulative volume of pores in the region where the pore size is largerthan the average major axis length of the power is, for instance, 1.0mL/g or less. The “cumulative volume of pores in a region where the porediameter is larger than the average major axis length of the powder” iscalled the “cumulative inter-particle void space” in this specification.

Moreover, the metal magnetic powder satisfies, for example, therelationship of Formula (1) between saturation magnetization σs (Am²/kg)and average particle volume including the oxide layer V (nm³):

σs≧0.0185V+58  (1).

The metal magnetic powder is preferably one whose rate of decrease insaturation magnetization Δσs when held for 1 week (168 h) in anatmosphere of a temperature of 60° C. and humidity of 90% RH is 15% orless.

The rate of decrease in saturation magnetization Δσs, which is an indexused to evaluate magnetic powder weatherability, is defined by thefollowing Formula (4):

Δσs (%)=(σs(i)−σs(ii))/(σs(i)×100  (4),

where σs(i) is the saturation magnetization (Am²/kg) before holding inthe aforesaid atmosphere and σs(ii) is the saturation magnetization(Am²/kg) after holding for 168 h in the aforesaid atmosphere.

Such a metal magnetic powder can be produced by subjecting the aforesaidmetal magnetic powder containing nonmagnetic constituents, in its stateafter firing or after both firing and high-temperature reduction, to aprocess using a complexing agent and a reducing agent in combination toelute the nonmagnetic constituents into a solution. More specifically,the present invention provides a method of manufacturing a magneticmetal powder for use in magnetic recording media comprising a step(elution process) carried out on a metal magnetic powder includingparticles having a metal magnetic phase composed mainly of Fe or Fe plusCo and containing one or more said nonmagnetic constituents, in whichstep a reducing agent acts in a solution added with a complexing agentcapable of forming a complex with one or more of said nonmagneticconstituents to elute nonmagnetic constituents from the powder particlesinto the solution. A step for forming an oxide layer on the surfaces ofthe powder particles (oxidation process) is preferably conducted afterthe elution process.

As the complexing agent can be used, for example, one or both ofdisodium tartrate and sodium citrate. As the reducing agent can be used,for example, one or more of hydrazine (N₂H₂), lithium aluminum hydride(LiAlH₄), sodium boron hydride (NaBH₄), and derivatives thereof.

The present invention enables an increase in the relative content of themetal element(s), i.e., the magnetized portion, while also achievingsinter prevention, not by reducing the amount of added sinteringinhibitor in the manufacture of a metal magnetic powder, but by means ofeluting from fired and high-temperature-reduced metal magnetic powdernonmagnetic constituents originating from sintering inhibitor that hascompleted its role. As a result, the present invention provides a metalmagnetic powder in which decrease of saturation magnetization withparticle-size refinement is markedly inhibited and which exhibits highersaturation magnetization than conventional metal magnetic powders forits particle size. Since sintering inhibitor is markedly removed fromthe particle surfaces, inter-particle bonds (necking) attributable tothe sintering inhibitor decouple to improve dispersibility. Further, nodecline in weatherability or other such harmful effect arises becausedecrease in saturation magnetization with particle-size refinement isinhibited even without implementing a measure such as making the oxidelayer especially thin. The present invention is therefore capable ofresponding to the recent ever more demanding need for enhancedhigh-recording density and weatherability (increased assurance ofsaturation magnetization being maintained at a high level) incoated-type magnetic recording media.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is graph showing how saturation magnetization σs varied as afunction of average particle volume V in metal magnetic powders obtainedin Examples and Comparative Examples.

FIG. 2 is a pore size distribution graph (with cumulative volume scaledon the vertical axis) for the metal magnetic powders of pre-elutionSample 3 and Example 5.

FIG. 3 is a diagram schematically representing the states of a metalmagnetic powder before and after elution processing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Metal Magnetic Powder

The magnetic powder that is the subject of this invention is a metalmagnetic powder whose particles have a metal magnetic phase composedmainly of Fe or Fe plus Co. More specifically, Fe or Fe plus Co accountfor 50 atomic % or greater of the magnetic elements (Fe, Co and Ni)constituting the metal magnetic phase. The powder particle surfaces arecoated with an oxide layer and the mol ratios among the elements presentin the whole particle including the oxide layer and the metal magneticphase are such that the ratio of Co to Fe (hereinafter called the Co/Featomic ratio) is 0-50 at. %. The Co/Fe atomic ratio is expressed as [Cocontent (at. %)/Fe content (at. %)×100]. The powder preferably has aCo/Fe atomic ratio of 5-45 at. %, more preferably 10-40 at. %. Withinthese ranges, stable magnetic properties can be readily obtained and theweatherability is also good. Although iron oxide is found in the oxidelayer, it is acceptable for oxides of other elements to be present aswell.

The “nonmagnetic constituent(s)” of rare earth element (Y being treatedas a rare earth element), Al, Si or other are added as sinteringinhibitor in the manufacturing process. However, since these nonmagneticconstituents are eluted from the metal magnetic powder by a method setout later in accordance with the present invention, the atomic ratio(R+Al+Si)/(Fe+Co) is kept to 20 at. % or less. That is, Formula (2) setout earlier is satisfied. At this time, a powder is afforded that has alarger saturation magnetization for its particle size than conventionalparticle-size-refined metal magnetic powders. Atomic ratio(R+Al+Si)/(Fe+Co) is more preferably 15 at. % or less, still morepreferably 13 at. % or less or 12 at. % or less.

The elements constituting the powder particles are not limited to Fe,Co, Ni, rare earth element (Y being treated as a rare earth element),Al, Si and O but may, for example, include alkaline earth metal elementadded as sintering inhibitor and various other elements. The compositionof the powder of this invention includes Fe, Co, Ni, rare earth element(Y being treated as a rare earth element), Al, Si and O, and the balanceof unavoidable impurities.

The range of acceptable content of nonmagnetic constituents originatingfrom sintering inhibitor is preferably defined based on the powdersurface area. Specifically, the nonmagnetic constituent content ispreferably not greater than 40 μmol/m² per unit surface area of thepowder, i.e., the nonmagnetic constituent content is preferably limitedso as to satisfy Formula(3) set out above. When the nonmagneticconstituent content increases to exceed this range, the saturationmagnetization of the metal magnetic powder tends to decrease and this inturn tends to degrade the C/N ratio of the magnetic recording medium.The total content of rare earth element, Al, Si and “X constituents”(herein X is defined as constituent elements other than Fe, Co, Ni, rareearth element (Y being treated as a rare earth element), Al, Si and O)is very preferably limited to not greater than 40 μmol/m², still morepreferably to not greater than 30 μmol/m², per unit surface area of thepowder.

The powder of this invention is composed of particles of a size suchthat the average major axis length is 10-50 nm and average particlevolume is 5,000 nm³ or less, preferably 4,500 nm³ or less. At a largerparticle size, magnetic tape recording density is difficult to enhanceto an adequately high level. The longest diameter of a particle imageobserved in a transmission electron micrograph is defined as the majoraxis length as termed with respect to this invention. On the other hand,when the photographically observed particle image is elliptical orcircular (when the ratio of the diameter of the longer portion (majordiameter) to the diameter of the shorter portion (minor diameter) isaround 1˜2, the length of the longest sectional diameter is defined asthe major axis length for the purpose at hand.

Thanks to the mitigation of inter-particle necking, the particlesaccording to this invention are characterized in having a cumulativeinter-particle void space of less than 1.0 mL/g. The “cumulativeinter-particle void space” as termed here is a value representing thesum of the pore volumes in the region where the pore size is larger thanthe average major axis length, when the pore size distribution wasmeasured by the mercury penetration method. To be more specific, what ismeasured as the pore size in the region where the pore diameter issmaller than the average major axis length is those of the pores presenton the particle surface itself, while what is measured in the regionwhere the pore diameter is larger than the average major axis lengthrepresents those of void space resulting from overlapping and adhesionof the particles.

The “average major axis length” is the average of the major axis lengthsof individual particles (at least 300 particles) measured from theaforesaid transmission electron micrograph.

Therefore, the reason for there being much void space in the regionwhere the pore diameter is larger than the average major axis length isthat there is a lot of inter-particle necking and a high probability ofcluster formation. Such clusters degrade packing at the time ofmedium-making and by extension are likely to lead to degradation ofmagnetic recording medium properties (increase in particulate noise anddegradation of squareness ratio and other orientation properties).Although this means that a smaller amount of void space is better, theinventors found that it is desirable for the cumulative inter-particlevoid space to be less than 1.0 mL/g, more preferably less than 0.9 mL/g,still more preferably less than 0.8 mL/g. While a smaller amount ofcumulative inter-particle void space is preferable, it is difficult toeliminate void space completely, because it occurs even when theparticles simply overlap, so that the calculated cumulativeinter-particle void space is always a value larger than 0 mL/g. Thecumulative inter-particle void space is also affected by the particleshape.

The particles can have any of various shapes, including acicular,spindle-like, flat-acicular-like and granular. The flat-acicular-likeshape is particularly preferable because overlapping of particles doesnot produce any void space, which results in an ideal structure withminimal dead space.

The magnetic properties of the invention metal magnetic powder mostpreferably satisfy the relationship of Formula (1) between saturationmagnetization σs (Am²/kg) and average particle volume including oxidelayer V (nm³):

σs≧0.0185V+58  (1).

Such a magnetic powder exhibits excellent saturation magnetization evenif small in particle size.

It is also desirable for the invention magnetic powder to beconcomitantly provided with excellent weatherability as demonstrated bya rate of decrease in saturation magnetization Δσs of 15% or less whenheld for 1 week in an atmosphere of a temperature of 60° C. and humidityof 90% RH.

The metal magnetic powder having these properties has very high utilityfor use in high-recording-density magnetic recording media.

Method of Manufacturing Metal Magnetic Powder

An ordinary method of manufacturing a metal magnetic powder can beadopted up to the stage of firing and reducing a starting powder addedwith sintering inhibitor. For example, iron oxy-hydroxide containing Coand sintering inhibitor is fired at a temperature of 250-700° C. by aknown method to convert it to α-Fe₂O₃ or other iron oxide. The ironoxide is then high-temperature reduced by gas-phase reduction to obtainmetal magnetic powder comprised mainly of α-Fe. This metal magneticpowder is called the Post-reduction Intermediate Product in the presentspecification. In order to obtain the metal magnetic powder of theinvention, it is necessary to subject the Post-reduction IntermediateProduct to a treatment for eluting nonmagnetic constituents originatingfrom the sintering inhibitor (elution process). Following the elutionprocess, the metal magnetic powder is formed with an oxide layer(oxidation process) to obtain the invention metal magnetic powder.

Elution Process

Although the Post-reduction Intermediate Product to be subjected to theelution process can be one whose particle surfaces are formed with anoxide layer, it is preferable for achieving more effective elution ofthe constituents originating from the sintering inhibitor to use apowder not formed with an oxide layer.

The processing solution is prepared by dissolution of a compound(complexing agent) capable of forming a complex with one or more of therare earth element (Y being treated as a rare earth element), Al and Sicontained in the Post-reduction Intermediate Product. While the choiceof complexing agent is not particularly limited, chemicals commonly usedas complexing agent in electroless plating, e.g., tartrate, citrate,malate and lactate, are advantageous for their ready availability. Theconcentration of the complexing agent should be made about 0.01-10mol/L. If necessary, a substance having a pH-buffering effect, e.g., anammonium salt or the like, can be added to the solution. The processingsolution can be prepared at around normal room temperature.

The Post-reduction Intermediate Product is added to the processingsolution. Addition of an excessive amount of the powder may causeheterogeneous reaction. Good results are generally obtained when theamount added is 1-100 g, preferably 5-50 g, per liter of processingsolution. The solution is preferably stirred or forcibly dispersed (suchas by ultrasonic dispersion) so as to keep the reaction uniform.

Once the powder has been uniformly dispersed in the processing solution,a reducing agent is added thereto. Hydrazine (N₂H₂), lithium aluminumhydride (LiAlH₄), sodium boron hydride (NaBH₄) or other such substanceknown to exhibit strong reducing power is used as the reducing agent.Use of a reducing agent of weak reducing ability is inadvisable becauseit tends to cause elution of the magnetic elements. Excessively high andlow reducing agent concentrations should be avoided because too high aconcentration degrades the nonmagnetic constituent eluting effect andtoo low a concentration tends to cause elution of magnetic elements. Thereducing agent concentration can usually be adjusted within the range of0.01-10 mol/L and is preferably adjusted to 0.05-5 mol/L, more desiredto 0.1-5 mol/L. Following addition of the reducing agent, leaching isperformed for 10-300 min with the solution temperature held at 10-50°C., preferably 15-40° C. This elutes nonmagnetic constituents into theprocessing solution, thereby increasing the relative content of themagnetic elements in the magnetic powder particles. This reaction ispreferably allowed to proceed in an inert gas atmosphere.

FIG. 3 schematically represents the states of a metal magnetic powderbefore and after the elution process. It is believed that in the powderbefore elution the sintering inhibitor causes the formation of a largenumber of bonds among the particles, thereby producing large voids amongthe particles. When many inter-particle void spaces are present, theybecome dead spaces within the magnetic layer so that magnetic particlepacking tends to be poor. On the other hand, in the powder afterelution, the dissolution and removal of the sintering inhibitor reducesthe number bonds to enhance the discreteness of the particles. Packingis therefore improved (the number of magnetic particles present per unitvolume of the magnetic layer is increased), so that a decrease in noiseand other effects can be expected at the time of medium-making.

Oxidation Process

The metal magnetic powder that has passed through the elution process isprocessed to form an oxide layer on the particle surfaces. The methodused is not particularly limited and a conventional method can beadopted. That is to say, the oxidation can be conducted by the wetmethod of casting an oxidizing agent into the solution used in theelution process or by the dry method of oxidizing powderseparated/extracted from the elution processing solution. In the drymethod, however, the powder is in an unstable state and must be handledwith caution.

Magnetic Recording Medium

The invention metal magnetic powder obtained in this manner can beutilized for the magnetic layer of a multilayer coated-type magneticrecording medium by an ordinary method.

The multilayer coated-type magnetic recording medium has a base film, anonmagnetic layer formed on the base film as an underlayer, and amagnetic layer formed on the nonmagnetic layer as an upper layer. Theinvention metal magnetic powder is blended into a coating compositionfor forming the upper magnetic layer.

The coating compositions for the underlayer and upper layer can both beprepared by the method of mixing the constituent materials in ratios toobtain the specified compositions and kneading/dispersing the mixturesusing a kneader and sand grinder. Application of the coatingcompositions to the base film is preferably done by the wet-on-wetmethod, which applies the upper magnetic layer as soon as possible whilethe underlayer is still wet.

The following can be given as an example of the structural elements ofthe multilayer coated-type magnetic recording medium.

Base Film

As examples of the base film can be cited resin films of polyesters suchas polyethylene terephthalate and polyethylene naphthalate, polyolefins,cellulose triacetate, polycarbonate, polyamide, polyimide, poly(amide-imide), polysulfone amide, and aromatic polyamide.

Coating Composition for Nonmagnetic Layer (Underlayer)

As an example of the nonmagnetic coating composition can be given onecomposed of 85 parts by mass of nonmagnetic powder (α-iron oxide,product of Dowa Mining Co., Ltd., average major axis particle diameter,80 nm), 20 parts by mass of carbon black, 3 parts by mass of alumina, 15parts by mass of vinyl chloride resin (MR-110 vinyl chloride-basebinder, product of Zeon Corp.), 15 parts by mass of polyurethane resin(UR-8200 polyurethane resin, product of Toyobo Co., Ltd.), 190 parts bymass of methyl ethyl ketone, 80 parts by mass of cyclohexanone, and 110parts by mass of toluene.

Coating Composition for Magnetic Layer (Upper Layer)

As an example of the magnetic coating composition can be given onecomposed 100 parts by mass of the invention metal magnetic powder, 5parts by mass of carbon black, 3 parts by mass of alumina, 15 parts bymass of vinyl chloride resin (MR-110, product of Zeon Corp.), 15 partsby mass of polyurethane resin (the aforesaid UR-8200), 1 part by mass ofstearic acid, 1 part by mass of acetylacetone, 190 parts by mass ofmethyl ethyl ketone, 80 parts by mass of cyclohexanone, and 110 parts bymass of toluene.

EXAMPLES Comparative Example 1

To 3,000 mL of pure water placed in a 5,000 ml beaker and maintained at40° C. using a temperature controller was added 500 mL of a solutionobtained by mixing a 0.03 mol/L solution of cobalt sulfate (reagentgrade) and a 0.15 mol/L aqueous solution of ferrous sulfate (reagentgrade) at a mixing ratio of 1:4. Next, granular sodium carbonate wasdirectly added in an amount such that the carbonate was 3 equivalentsrelative to (Fe+Co) to prepare a suspension composed mainly of ferrouscarbonate while controlling the liquid temperature so as not to varyexceed ±5° C. from 40° C. The suspension was ripened for 1.5 hour, addedwith air at the rate of 50 mL/min in an amount adjusted to make the Feion oxidation rate 20%, thereby forming crystal nuclei, heated to 65°C., and aerated with pure oxygen at 50 mL/min to continue oxidation for1 hr.

The liquid temperature was then lowered to 40° C. and after thetemperature stabilized, an aqueous aluminum sulfate solution of 1.0 mass% as Al was continuously added at the rate of 5.0 g/min for 20 min togrow iron oxy-hydroxide. Pure oxygen was further passed at 50 mL/min tocomplete oxidation. Oxidation was determined to be complete followingthe point when a small sample of supernatant showed no change in coloron testing with potassium hexacyanoferrate.

The liquid following completion of oxidation was added with 300 g of asolution of yttrium oxide in aqueous sulfuric acid (containing 2.0 mass% as Y). By this, Al entered solid solution and an iron oxy-hydroxidepowder having Y adhered to its surface was obtained.

The iron oxy-hydroxide cake was separated by ordinary filtering, washedwith water and dried at 130° C. to obtain iron oxy-hydroxide in the formof a dry solid. Ten grams of the solid was placed in a bucket and firedin air at 400° C. under addition of steam at an introduction rate of 1.0g/min (water basis), thereby affording an iron-system oxide composedmainly of α-iron oxide (hematite).

The α-iron oxide was placed in a gas-permeable bucket and the bucket wasloaded into a penetrate-type reduction furnace. The α-iron oxide wasreduced by firing for 30 min at 400° C. with passage of hydrogen gas(flow rate: 40 L/min) and addition of steam at an introduction rate of1.0 g/min (water basis). At the end of the reduction period, the supplyof steam was terminated and the temperature was elevated to 600° C. at atemperature increase rate of 10° C./min in the presence of a hydrogenatmosphere. High-temperature reduction was then conducted for 60 minunder addition of steam at an introduction rate of 1.0 g/min (waterbasis) to produce a metal magnetic powder (iron-system alloy powder).The metal magnetic powder at this stage had not yet been processed(oxidized) to form an oxide layer and corresponded to the aforesaidPost-reduction Intermediate Product.

Oxidation Process

In this Comparative Example, the Post-reduction Intermediate Product wasformed with an oxide layer using a conventional oxidation process.Specifically, the intermediate product was transferred to the oxidationprocess without removing it from the bucket. That is, the atmosphere inthe furnace was then changed from hydrogen to nitrogen and the furnacetemperature was lowered to 90° C. at a temperature decrease rate of 20°C./min while introducing nitrogen at a flow rate of 50 L/min. At theinitial stage of oxide layer formation, a mixed gas obtained bysupplying nitrogen at 50 L/min and pure oxygen at 400 mL/min was addedto the furnace while also adding steam at an introduction rate of 1.0g/min (water basis), thereby forming oxide layer in a mixed atmosphereof steam, oxygen and nitrogen. At the point where heat generation bysurface oxidation subsided, the amount of oxygen supplied was graduallyincreased to raise the oxygen concentration of the atmosphere. The finalpure oxygen flow rate was made 2.0 L/min. At this time, the total amountof gas introduced into the furnace was held substantially constant byregulating the nitrogen flow rate. Stabilization was carried out in anatmosphere maintained at about 90° C.

The powder properties and composition of the metal magnetic powder(after oxidation) obtained in this manner were examined as set outbelow.

Measurement of Major Axis Length and Minor Axis Length

A JEM-100CX MARK-II Transmission Electron Microscope (product of JEOL)was used at an acceleration voltage of 100 kV to observe a bright-fieldimage of the powder to be measured. The observed image was photographedat a magnification of, say, 58000× and enlarged at a magnification of,say, 9× in both width and length. For each sample, 300 particles wereselected at random from among mono-dispersed particles in a number ofphotographic images, and the major axis length and minor axis lengthappearing in the photographic images were measured for each selectedparticle. The averages of the measured major and minor axis lengths wererespectively defined as the major axis length and minor axis length ofthe sample.

Particle Volume

The average values of the major axis length and minor axis lengthmeasured in the foregoing manner were used to calculate particle volumeby cylindrical approximation in accordance with the following equation:

(Particle volume)=π×(Major axis length)×(Minor axis length/2)²

Specific Surface Area

Specific surface area was determined by the BET method using aQuadrasorb US instrument (product of Yuasa-Ionics).

Crystallite Size

Crystallite size was calculated from the following Formula (5) usingdata obtained with an X-ray diffractometer (Rigaku RAD-2C):

Crystallite size=Kλ/βcos θ  (5),

where K is the Scherrer constant=0.9, λ is the wavelength of the Co-KαX-ray emission line, β is the half-value of the Fe(110) diffraction peak(radian), and θ is the diffraction angle (radian).

The measurement range scanned for the calculation was 20=45-60°. Thescanning speed was 5°/min and the number of integrations was 5.

Magnetic Properties and Weatherability

The magnetic properties of the powder were measured under a 10 kOe(795.8 kA/m) external magnetic field using a VSM-7P vibrating samplemagnetometer (product of Toei-Kogyo Co., Ltd.). The sample powder wasstored for one week in a thermohygrostat at a temperature of 60° C. andhumidity of 90% RH, the change in the saturation magnetization Δσsbefore and after storage in the thermohygrostat was measured, and theweatherability of the powder was calculated in accordance with Formula(4).

Composition Analysis of Powder Particles

The composition of the powder particles was determined by performingmass analysis on the entire particle including both the metal magneticphase and the oxide layer. Determination of Co, Al and rare earthelement (Y being treated as a rare earth element) was done using anIris/AP High-frequency Inductively Coupled Plasma Spectrometer (productof Jarrell Ash Japan), Fe determination was done using a HiranumaAutomatic Titrator (COMTIME-980; product of Hiranuma Sangyo Co., Ltd.),and oxygen determination was done using a Nitrogen/Oxygen Determinator(TC-436; product of LECO Corporation). As the determination results wereobtained in mass percent, they were suitably converted to atomic percent(at. %) to obtain the Co/Fe atomic ratio, Al/(Fe+Co) atomic ratio,Y/(Fe+Co) atomic ratio, and (R+Al+Si)/(Fe+Co) atomic ratio. In theComparative Examples and Examples, Si/(Fe+Co) was below the measurementlimit, so that in these examples (R+Al+Si)/(Fe+Co) atomic ratio was thesame as (R+Al)/(Fe+Co) atomic ratio.

The results are shown in Table 1 (along with those for the ComparativeExamples set out below).

Comparative Examples 2-24

A manufacturing method like that of Comparative Example 1 was used toproduce powders of various compositions and particle sizes by varyingthe composition and oxidation conditions.

The compositions and magnetic properties of the samples obtained areshown in Table 1.

Example 1

A Post-reduction Intermediate Product was produced by a manufacturingmethod like that of Comparative Example 1. The product obtained bysubjecting the Post-reduction Intermediate Product to the “oxidationprocess” set out in Comparative Example 1 was designated Pre-elutionSample 1. The powder properties and magnetic properties of thePre-elution Sample 1 were determined by the same methods as inComparative Example 1. The results are shown in Table 2 (along withthose for Pre-elution Samples 2 and 3 set out below).

The metal magnetic powder before the Pre-elution Sample 1 was oxidized,i.e., the Post-reduction Intermediate Product, was subjected to elutionin the following manner.

A mixture containing 0.05 mol/L of disodium tartrate as complexing agentand 0.1 mol/L of ammonium sulfate as buffer was adjusted to pH 9 withNH₃ to prepare a processing solution. 10 g of the Pre-elution Sample 1metal magnetic powder was cast into the processing solution, which wasmaintained at 30° C., followed by addition of sodium tetrahydroborate asreducing agent to a concentration of 0.3 mol/L. The result was ripenedat 30° C. for 30 min under stirring to obtain a slurry. The slurry wassubjected to solid-liquid separation to obtain a solids component and afiltrate.

The solids component was filtered, washed with water and dried to obtaina dry product. The dry product was oxidized under oxidation conditionssimilar to those shown in Comparative Example 1 to obtain Example 1Metal Magnetic Powder. The powder properties and composition of theExample 1 Metal Magnetic Powder were examined in the manner ofComparative Example 1.

On the other hand, the filtrate was measured for the concentration (ppm)of Fe, Co, Al and Y as components eluted from the powder.

The results are shown in Table 2 (along with those for the Examples setout below). “Example 1 Metal Magnetic Powder” is abbreviated to “Example1” in Table 2. Similar abbreviations are used with regard to theExamples set out below. The terms “Major axis length” and “Minor axislength” appearing in Table 2 mean “Average major axis length” and“Average minor axis length,” respectively.

Next, Pre-elution Sample 1 and Example 1 Metal Magnetic Powder were usedto fabricate magnetic tapes by the following method and the tapes weretested for their magnetic properties as recording media. In order tomore clearly ascertain the effects of the metal magnetic powders, thetapes were fabricated to have only a magnetic layer, without provisionof a nonmagnetic layer.

(1) Magnetic Coating Composition Preparation

Magnetic powder, 0.35 g, was weighed out and placed in a pot (insidediameter: 45 mm, depth: 13 mm) and allowed to stand for 10 min. with thecover open. Next, 0.700 mL of a vehicle [mixed solution of vinylchloride resin MR-10 (22 mass %, product of Zeon Corp.), cyclohexanone(38.7 mass %), acetylacetone (0.3 mass %), n-butyl stearate (0.3 mass %)and methyl ethyl ketone (38.7 mass %)] was added to the pot using amicropipette. A 30 g steel ball (2φ) and ten nylon balls (8φ) wereimmediately added to the pot and the pot was covered and allowed tostand for 10 min. The pot was then set in a centrifugal ball mill(Fritsch P-6) and the rotating speed was gradually raised to 600 rpm, atwhich dispersion was continued for 60 min. The centrifugal ball mill wasstopped and the pot removed. Using a micropipette, the pot was addedwith 1.800 mL of a liquid adjuster prepared in advance by mixing methylethyl ketone and toluene at a ratio of 1:1. The pot was again set in thecentrifugal ball mill and rotated at 600 rpm for 5 minutes. Thiscompleted the dispersion.

(2) Magnetic Tape Preparation

Upon completion of the foregoing dispersion, the cover of the pot wasopened and the nylon balls removed. The coating composition, togetherwith the steel ball, was placed in an applicator (55 μm) and coated ontoa base film (15 μm polyethylene film marketed by Toray Industries underthe product designation 15C-B500). The coated film was promptly placedat the center of the coil of a 5.5 kG magnetic orientation device toorient its magnetic field, and then dried.

(3) Tape Property Evaluation

The coercive force Hcx, switching field distribution SFD, squrarenessratio SQ and orientation ratio OR of the obtained tapes were measuredusing the aforesaid VSM.

The properties of the obtained tapes are shown in Table 3.

Next, the pore size distribution of the metal magnetic powders ofPre-elution Sample 1 and Example 1 Metal Magnetic Powder were measuredas set out below.

Pore Size Distribution

Pore size distribution was measured by the mercury penetration methodusing a pore distribution porosity meter (AutoPore IV 9500 V1.05,product of Micrometrics Instrument Corporation).

The aforesaid “cumulative inter-particle void space” was calculated fromthe pore size distribution and the value of the average major axislength. A powder that had a “cumulative inter-particle void space” valueof less than 1.0 mL/g was rate G (Good) and one that had a value of 1.0mL/g or greater was rated P (Poor, i.e. of the conventional level). Theresults are shown in Table 3 (along with those for the Examples set outbelow).

Example 2

The metal magnetic powder before the Pre-elution Sample 1 was oxidized,i.e., the Post-reduction Intermediate Product, was subjected to elutionunder the following conditions.

A mixture containing 0.05 mol/L of sodium citrate as complexing agentand 0.1 mol/L of ammonium sulfate as buffer was adjusted to pH 9 withNH₃ to prepare a processing solution. 10 g of the Pre-elution Sample 1metal magnetic powder was cast into the processing solution, which wasmaintained at 30° C., followed by addition of hydrazine as reducingagent to a concentration of 4 mol/L. The result was ripened at 30° C.for 30 min under stirring to obtain a slurry. The slurry was subjectedto solid-liquid separation and the properties of the metal magneticpowder were examined in the manner of Example 1.

Example 3

A Post-reduction Intermediate Product differing in composition from thePre-elution Sample 1 was produced by a method similar to that describedin Comparative Example 1. The result was subjected to the “oxidationprocess” of Comparative Example 1 to obtain a “Pre-elution Sample 2.”

The metal magnetic powder before the Pre-elution Sample 2 was oxidized,i.e., the Post-reduction Intermediate Product, was subjected to elutionunder the conditions of Example 1. The slurry was subjected tosolid-liquid separation and the properties of the metal magnetic powderwere examined in the manner of Example 1.

Example 4

The metal magnetic powder before the Pre-elution Sample 2 was oxidized,i.e., the Post-reduction Intermediate Product, was subjected to elutionunder the conditions of Example 2. The slurry was subjected tosolid-liquid separation and the properties of the metal magnetic powderwere examined in the manner of Example 1.

Example 5

A Post-reduction Intermediate Product differing in composition from thePre-elution Samples 1 and 2 was produced by a method similar to thatdescribed in Comparative Example 1. The result was subjected to theoxidation process of Comparative Example 1 to obtain a “Pre-elutionSample 3.”

The metal magnetic powder before the Pre-elution Sample 3 was oxidized,i.e., the Post-reduction Intermediate Product, was subjected to elutionunder the following conditions.

A processing solution was prepared as a mixture containing 0.0028 mol/Lof disodium tartrate as complexing agent and 0.0056 mol/L of ammoniumsulfate as buffer. 10 g of the Pre-elution Sample 3 metal magneticpowder was cast into the processing solution, which was maintained at30° C., followed by addition of sodium boron hydride as reducing agentto a concentration of 0.004 mol/L. The result was ripened at 30° C. for30 min under stirring to obtain a slurry. The slurry was subjected tosolid-liquid separation and the properties of the metal magnetic powderwere examined in the manner of Example 1.

TABLE 1 BET Major Minor Average Specific Saturation Coercive Weather-axis axis particle surrace Crystallite magnetization force abilitylength length volume V area size σs Hc Δσs No. (nm) (nm) (nm³) (m²/g)(nm) (Am²/kg) (kA/m) (%) Comparative Example 1 36.1 10.4 3067 73 11 109172.3 9 Comparative Example 2 33.9 8.7 2015 97 8 89 154.4 11 ComparativeExample 3 34.9 9.5 2474 91 9 95 159.6 13 Comparative Example 4 35.1 10.22868 74 10 107 142.0 9 Comparative Example 5 37.0 10.0 2906 73 11 106168.3 9 Comparative Example 6 37.5 10.2 3064 74 11 108 163.9 9Comparative Example 7 37.0 9.9 2848 80 9 100 177.5 13 ComparativeExample 8 35.4 9.8 2670 84 9 102 168.7 13 Comparative Example9 37.0 11.03516 67 11 119 174.7 10 Comparative Example 10 45.8 11.1 4432 70 12 116192.2 10 Comparative Example 11 44.8 10.2 3661 75 10 109 175.5 10Comparative Example 12 46.7 11.3 4683 68 12 123 196.2 8 ComparativeExample 13 43.7 9.7 3229 78 10 103 179.0 11 Comparative Example 14 46.110.1 3693 75 10 107 180.2 10 Comparative Example 15 46.2 10.3 3850 75 10109 177.5 10 Comparative Example 16 43.7 9.7 3229 78 10 103 179.0 11Comparative Example 17 46.8 11.0 4448 73 11 107 184.6 6 ComparativeExample 18 44.6 10.8 4086 75 11 108 187.0 11 Comparative Example 19 45.210.9 4218 74 11 111 187.8 11 Comparative Example 20 45.5 10.9 4246 73 11113 188.6 10 Comparative Example 21 43.2 12.1 4968 58 13 120 187.4 6Comparative Example 22 46.4 11.0 4410 70 11 113 195.8 11 ComparativeExample 23 46.9 10.3 3908 75 11 110 176.7 10 Comparative Example 24 43.710.0 3432 75 11 110 172.7 12 Atomic ratio *1 (R + Al + Si) Al/ Y/ (R +Al + Si)/ per unit Co/Fe (Fe + Co) (Fe + Co) (Fe + Co) surface area No.(at %) (at %) (at %) (at %) (μmol/m²) Comparative Example 1 24 14.8 15.029.8 70.9 Comparative Example 2 27 19.8 12.3 32.1 57.2 ComparativeExample 3 27 19.8 12.3 32.1 61.0 Comparative Example 4 24 14.8 15.0 29.870.0 Comparative Example 5 24 14.8 15.0 29.8 70.9 Comparative Example 624 13.1 12.8 25.9 60.7 Comparative Example 7 24 18.3 12.1 30.4 65.7Comparative Example 8 24 20.1 11.8 31.9 65.5 Comparative Example9 2413.1 12.8 25.9 67.0 Comparative Example 10 24 14.4 7.8 22.2 54.7Comparative Example 11 24 14.4 7.8 22.2 51.0 Comparative Example 12 2413.1 7.8 20.9 53.0 Comparative Example 13 24 14.4 12.5 26.9 59.7Comparative Example 14 24 14.4 12.5 26.9 62.1 Comparative Example 15 2414.4 7.8 22.2 51.0 Comparative Example 16 24 14.4 12.5 26.9 59.7Comparative Example 17 24 14.4 7.8 22.2 52.4 Comparative Example 18 2414.4 7.8 22.2 51.0 Comparative Example 19 24 14.4 7.8 22.2 51.7Comparative Example 20 24 14.4 7.8 22.2 52.4 Comparative Example 21 2410.5 12.7 23.2 69.5 Comparative Example 22 24 13.1 12.5 25.6 63.4Comparative Example 23 24 14.4 7.8 22.2 51.0 Comparative Example 24 2410.2 12.5 22.7 52.7 *1 R: Rare earth elament (Y being treated as a rareearth elament)

TABLE 2 Sat- BET uration Atomic ratio *1 Major Minor Average SpecificCrystal- magneti- Coercive Weather- (R + axis axis particle surface litezation force ability Al/ Y/ Al + Si)/ length length volume V area sizeσs Hc Δσs Co/Fe (Fe + Co) (Fe + Co) (Fe + Co) No. (nm) (nm) (nm³) (m²/g)(nm) (Am²/kg) (kA/m) (%) (at %) (at %) (at %) (at %) Pre-elution 38.511.6 4100 90 10.2 101 144.3 15 24 12.5 11.9 24.4 Sample 1 Example 1 29.99.4 2075 84 10.1 108 129.6 13 24 5.9 5.8 11.7 Example 2 30.4 9.8 2293 7410.1 114 141.0 10 24 5.0 5.1 10.1 Pre-elution 44.0 11.3 4413 81 11.3 108173.6 15 24 13.1 8.2 21.3 Sample 2 Example 3 36.7 9.6 2656 73 11.3 117162.6 9 24 6.1 4.0 10.1 Example4 36.0 9.7 2660 77 11.0 112 156.0 9 246.3 4.2 10.5 Pre- 38.2 11.0 3630 103 9.5 87 152.2 15 24 17.6 11.2 28.8elution 3 Example 5 30.9 9.4 2144 93 9.5 106 155.0 15 24 6.2 6.9 13.1(R + Al + Si) Concentration of per unit (R + Al + Si) components elutedinto filtrate surface area reduction ratio Fe Co Al Y No. (μmol/m²) (%)(ppm) (ppm) (ppm) (ppm) Pre-elution Sample 1 47.0 — — — — — Example 124.1 48.6 40 5 200 620 Example 2 11.5 75.6 50 6 220 710 Pre-elutionSample 2 45.3 — — — — — Example 3 23.8 47.5 40 5 200 400 Example4 13.969.2 50 5 200 410 Pre-elution 3 29.5 — — — — — Example 5 17.2 41.5 10 7340 410 *1 R: Rare earth elament (Y being treated as a rare earthelament)

TABLE 3 Cumulative inter-particle Tape properties void Hc No. space *1(kA/m) (Oe) SFD SQ OR Pre-elution Sample P 171.3 2152 0.83 0.80 2.00 1Example 1 G 184.2 2315 0.70 0.83 2.70 Example 2 G 194.6 2445 0.72 0.812.50 Pre-elution Sample P 203.2 2553 0.57 0.85 2.30 2 Example 3 G 218.92751 0.50 0.86 2.70 Example 4 G 217.1 2728 0.47 0.87 2.70 Pre-elutionSample P 177.3 2228 0.76 0.79 2.00 3 Example 5 G 198.1 2490 0.62 0.852.70 *1 G (Good): Less than 1.0 mL/g P (Poor): 1.0 mL/g or greater

It will be noted that in the metal magnetic powders indicated asExamples, the elution carried out on the powders markedly reduced thecontent of nonmagnetic constituents (Y, Al) originating from thesintering inhibitor. The amounts of Fe and Co washed out by the elutionwere slight in comparison with the amounts of Al and Y. Thisdemonstrates that the amounts of the nonmagnetic constituents containedcould be reduced selectively and efficiently.

FIG. 1 shows how saturation magnetization σs varied as a function ofaverage particle volume V. It can be seen that the elution carried outon the invention metal magnetic powders obtained in the Examples enabledthem to satisfy Formula (1) and to achieve greatly improved saturationmagnetization σs for their small size in comparison with the metalmagnetic powders of the Comparative Examples. In other words, it wasfound that the invention can provide a metal magnetic powder for use inmagnetic recording media that is improved in the relationship betweenits particle size and its magnetic properties. In addition, theweatherability of the invention metal magnetic powder, judged based onthe rate of decrease in saturation magnetization Δσs, was excellent andon a par with or superior to the conventional level.

As shown in Table 3, the powders of the Examples after completion ofelution were reduced in cumulative inter-particle void space to lessthan 1.0 mL/g and were found to have concomitantly achieved a markedimprovement in magnetic properties. This is believed to be attributableto the fact that the orientation property in the medium was improvedowing to elimination of inter-particle necking on a major scale.

FIG. 2 is a pore size distribution graph (with cumulative volume scaledon the vertical axis) showing plots for the metal magnetic powders ofPre-elution Sample 3 and Example 5.

1. A metal magnetic powder for a magnetic recording medium whoseparticles have a metal magnetic phase, composed mainly of Fe or Fe plusCo, and an oxide layer, wherein the average major axis length of thepowder particles is 10-50 nm, the average particle volume including theoxide layer is 5,000 nm³ or less, and the atomic ratio (R+Al+Si)/(Fe+Co)calculated using the content values (at. %) of the elements contained inthe powder particles is 20% or less, where R is rare earth element (Ybeing treated as a rare earth element).
 2. The metal magnetic powder fora magnetic recording medium according to claim 1, wherein the totalcontent of rare earth element (Y being treated as a rare earth element),Al and Si in the powder particles is not greater than 40 μmol/m² perunit surface area of the powder.
 3. The metal magnetic powder for amagnetic recording medium according to claim 1, wherein when its poresize distribution is measured by the mercury penetration method, themetal magnetic powder has such relationship that the cumulative volumeof pores in the region where the pore size is larger than the averagemajor axis length of the power is 1.0 mL/g or less.
 4. The metalmagnetic powder for a magnetic recording medium according to claim 1,which metal magnetic powder satisfies the relationship of Formula (1)between saturation magnetization σs (Am²/kg) and average particle volumeincluding the oxide layer V (nm³) and whose rate of decrease insaturation magnetization Δσs when held for 1 week in an atmosphere of atemperature of 60° C. and humidity of 90% RH is 15% or less:σs≧0.0185V+58  (1).
 5. A method of manufacturing the metal magneticpowder for a magnetic recording medium defined by claim 1, comprising: astep (elution process) of subjecting a metal magnetic powder includingparticles having a metal magnetic phase composed mainly of Fe or Fe plusCo and containing a nonmagnetic constituent composed of one or more ofrare earth element (Y being treated as a rare earth element), Al and Si,in which step a reducing agent acts in a solution added with acomplexing agent capable of forming a complex with one or more of saidnonmagnetic constituent to elute the nonmagnetic constituent from thepowder particles into the solution.
 6. The method of manufacturing themetal magnetic powder for a magnetic recording medium according to claim5, wherein one or both of disodium tartrate and sodium citrate is usedas the complexing agent.
 7. The method of manufacturing the metalmagnetic powder for a magnetic recording medium according to claim 5,wherein one or more of hydrazine (N₂H₂), lithium aluminum hydride(LiAlH₄), sodium boron hydride (NaBH₄), and derivatives thereof are usedas the reducing agent.
 8. The method of manufacturing the metal magneticpowder for a magnetic recording medium according to claim 5, furthercomprising after the elution process: a step (oxidation process) forforming an oxide layer on the surfaces of the powder particles.
 9. Amagnetic recording medium using the metal magnetic powder according toany of claims 1 to claim
 1. 10. The metal magnetic powder for a magneticrecording medium according to claim 2, wherein when its pore sizedistribution is measured by the mercury penetration method, the metalmagnetic powder has such relationship that the cumulative volume ofpores in the region where the pore size is larger than the average majoraxis length of the power is 1.0 mL/g or less.
 11. The metal magneticpowder for a magnetic recording medium according to claim 2, which metalmagnetic powder satisfies the relationship of Formula (1) betweensaturation magnetization σs (Am²/kg) and average particle volumeincluding the oxide layer V (nm³) and whose rate of decrease insaturation magnetization Δσs when held for 1 week in an atmosphere of atemperature of 60° C. and humidity of 90% RH is 15% or less:σs≧0.0185V+58  (1).
 12. The metal magnetic powder for a magneticrecording medium according to claim 3, which metal magnetic powdersatisfies the relationship of Formula (1) between saturationmagnetization σs (Am²/kg) and average particle volume including theoxide layer V (nm³) and whose rate of decrease in saturationmagnetization Δσs when held for 1 week in an atmosphere of a temperatureof 60° C. and humidity of 90% RH is 15% or less:σs≧0.0185V+58  (1).
 13. A method of manufacturing the metal magneticpowder for a magnetic recording medium defined by claim 2, comprising: astep (elution process) of subjecting a metal magnetic powder includingparticles having a metal magnetic phase composed mainly of Fe or Fe plusCo and containing a nonmagnetic constituent composed of one or more ofrare earth element (Y being treated as a rare earth element), Al and Si,in which step a reducing agent acts in a solution added with acomplexing agent capable of forming a complex with one or more of saidnonmagnetic constituent to elute the nonmagnetic constituent from thepowder particles into the solution.
 14. A method of manufacturing themetal magnetic powder for a magnetic recording medium defined by claim3, comprising: a step (elution process) of subjecting a metal magneticpowder including particles having a metal magnetic phase composed mainlyof Fe or Fe plus Co and containing a nonmagnetic constituent composed ofone or more of rare earth element (Y being treated as a rare earthelement), Al and Si, in which step a reducing agent acts in a solutionadded with a complexing agent capable of forming a complex with one ormore of said nonmagnetic constituent to elute the nonmagneticconstituent from the powder particles into the solution.
 15. A method ofmanufacturing the metal magnetic powder for a magnetic recording mediumdefined by claim 4, comprising: a step (elution process) of subjecting ametal magnetic powder including particles having a metal magnetic phasecomposed mainly of Fe or Fe plus Co and containing a nonmagneticconstituent composed of one or more of rare earth element (Y beingtreated as a rare earth element), Al and Si, in which step a reducingagent acts in a solution added with a complexing agent capable offorming a complex with one or more of said nonmagnetic constituent toelute the nonmagnetic constituent from the powder particles into thesolution.
 16. The method of manufacturing the metal magnetic powder fora magnetic recording medium according to claim 6, wherein one or more ofhydrazine (N₂H₂), lithium aluminum hydride (LiAlH₄), sodium boronhydride (NaBH₄), and derivatives thereof are used as the reducing agent.17. The method of manufacturing the metal magnetic powder for a magneticrecording medium according to claim 6, further comprising after theelution process: a step (oxidation process) for forming an oxide layeron the surfaces of the powder particles.
 18. The method of manufacturingthe metal magnetic powder for a magnetic recording medium according toclaim 7, further comprising after the elution process: a step (oxidationprocess) for forming an oxide layer on the surfaces of the powderparticles.
 19. A magnetic recording medium using the metal magneticpowder according to claim
 2. 20. A magnetic recording medium using themetal magnetic powder according to claim
 3. 21. A magnetic recordingmedium using the metal magnetic powder according to claim 4.